The observations of type Ia supernovae show that the expansion of the Universe is accelerating. The acceleration requires the Universe to be dominated by an exotic fluid with negative pressure. The simplest explanation for dark energy is the cosmological constant or vacuum energy that explains the acceleration of the Universe. Though the cosmological constant is preferred from cosmological observations yet its theoretical understanding has been questionable [1]}. The other alternatives of the cosmological constant that can act as dark energy are scalar fields such as quintessence field, modified gravity, phantom dark energy, etc [2]}. There has not been any observational evidence of such alternatives but it has not been ruled out either. One such model is dynamical dark energy driven by a slowly rolling scalar field. If this is true, then it opens up many new observational windows which may shed light on the fundamental nature of this mysterious component of the Universe.
